1. Field of the Invention
The invention relates in general to a circuit board module and a forming method thereof, and more particularly to a circuit board module capable of increasing the mechanical strength between two electrically connected flexible printed circuit boards and reducing the manufacturing cost and a forming method thereof.
2. Description of the Related Art
The flexible circuit board at least includes an insulated base, an adhering layer and a copper circuit. The flexible circuit board possesses the characteristics of flexibility, lightweight and thinness can be distributed three-dimensionally and embedded into a processed conductor freely. Due to the flexibility of the flexible circuit board, the circuit designer can use the flexible circuit board for three-dimensional distribution to fit the space design and the bending shapes of various electronic products.
The flexible circuit board is basically categorized as the single-layered flexible circuit board and the double-layered flexible circuit board. The single-layered flexible circuit board refers to the flexible circuit board with a single-layered circuit, and has only one layer of copper circuit. The double-layered flexible circuit board refers to the flexible circuit board with a double-layered circuit. The double-layered circuit has two layers of copper circuit. In the single-layered flexible circuit board, the copper circuit is disposed on the insulated base via an adhering layer, and the passivation layer covers part of the copper circuit and exposes the bare copper as a solder pad. In the double-layered flexible circuit board, the first copper circuit and the second copper circuit are respectively disposed on the top surface and the bottom surface of the base via respective adhering layer, and are electrically connected to each other via a conductive buried hole penetrating the base and the adhering layer. The first copper circuit and the second copper circuit are respectively covered by a passivation layer cover with a bare copper being exposed as a solder pad. Therefore, the flexible circuit board can be soldered to the solder pad of other circuit boards or other electronic elements via the bare copper.
Take the liquid crystal display (LCD) device for example. The control circuit board of the light emitting diode (LED) of a backlight module is a double-layered flexible circuit board. One of the solder pads has an LED, and the other solder pad can be soldered to the solder pad of the control circuit board of the liquid crystal display (LCD) panel. Therefore, the LED and the solder pad connected externally are positioned on different sides of the flexible circuit board. Besides, examples of the control circuit board of the LCD panel include a double-layered flexible circuit board.
Conventionally, if two flexible circuit boards are soldered according to the pressure soldering method to form a circuit board module, a double-layered flexible circuit board is most likely to be used. That is, the solder pads of the two double-layered flexible circuit boards are soldered together face to face via a solder, so that the two double-layered flexible circuit boards are electrically connected. However, the double-layered flexible circuit board is costive, so the manufacturing cost can not be reduced.
If the two flexible circuit boards are soldered together according to the draw soldering method, one of the two flexible circuit boards can be a single-layered flexible circuit board, which uses the bare copper as a solder pad to be soldered to the solder pad of the other flexible circuit board. Prior to the step of draw soldering, the single-layered flexible circuit board is disposed on the other flexible circuit board, so that the solder pad of the single-layered flexible circuit board and the solder pad of the other flexible circuit board face the same direction. One lateral side of the single-layered flexible circuit board has a solder pad, and the lateral side of the single-layered flexible circuit board covers part of the solder pad of the other flexible circuit board. Then, the draw soldering method is applied to solder the solder pad of the single-layered flexible circuit board to the solder pad of the other flexible circuit board, so that the single-layered flexible circuit board is electrically connected to the other flexible circuit board.
However, when defective soldering occurs in a conventional draw soldering method, empty soldering would occur beside a lateral side of the single-layered flexible circuit board, causing the mechanical strength connecting the single-layered flexible circuit board and the other flexible circuit board to be weakened significantly. Under the interference of an external force, the bare copper would crack easily and the copper circuit would be disconnected easily. In a worse case, peeling would occur between the single-layered flexible circuit board and the other flexible circuit board, largely reducing the reliability and practicality of the circuit board module and increasing the manufacturing cost. In order to avoid empty soldering, more time needs to be spent on draw soldering. As disclosed above, the single-layered flexible circuit board adopting the draw soldering method has several disadvantages. In actual application, the double-layered flexible circuit board is more commonly used. However, using the double-layered flexible circuit board would increase the manufacturing cost. Regardless whether the single-layered flexible circuit board or the double-layered flexible circuit board is used, the pressure soldering method and the draw soldering method have their own problems to be tackled in forming a circuit board module.